Sparse arrays in R: the spray package

In this short article I introduce the spray package, which provides some functionality for handling sparse arrays. The package uses the C++ Standard Template Library's map class to store and retrieve elements. One natural application for sparse arrays is multivariate polynomials and I give two examples of the package in use, one drawn from the fields of random walks on lattices and one from the field of recreational combinatorics. The package is available on CRAN at https://CRAN.R-project.org/package=spray.